Vacuum impregnation process



y 1951 H. M. KROFT 2,554,254

VACUUM IMPREGNATION PROCESS Filed June 16, 1949 WITNESSES: INVENTORHerbert M. Kroft.

Patented May 22, 1951 UNITED STATES ATENT OFFICE VACUUM IMPREGNATIONPROCESS Application June 16, 1949, Serial No. 99,488

Claims.

This invention relates to the vacuum impregnation of porous members withfluid compositions.

In treating members having pores, crevices or other fine voids,hereinafter designated porous members, with a fluid composition so as toimpregnate the pores of the members thoroughly and completely with thefluid composition, it is well known that considerable difiiculty isexperienced in securing such complete impregnation. Even with thebenefit of a preliminary evacuation of the porous members, it has beenfound in many cases that the fluid compositions fail to penetrate to theinnermost portions of the pores and spaces of porous members.

Thus in the manufacture of electrical coils, it is desirable tothoroughly saturate the fibrous insulation, such as paper, cloth,silver; or felt, commonly applied both to the conductors and the coilwith an insulating varnish. To this end electrical coils are placed invacuum tanks wherein they are evacuated to a low absolute pressure towithdraw air, moisture and other volatiles from the interstices of thecoils, and thereafter the coils are flooded with insulating varnishcomposition, and pressures above atmosphere are then applied to thevarnish composition to drive it into the interstices of the coil. Inmany cases, particularly with the larger coils wrapped with manylayersof tape, the varnish does not completely penetrate the entireinsulation. The innermost layers often have very little impregnantpresent. Likewise, in impregnating electrical members with insulatingoils, it has been found that the penetration of the oil has beenunsatisfactory even with the assistance of a vacuum. This incompleteimpregnation is particularly prevalent where the member being treatedhas extremely fine pores or crevices some of which have been found to beunimpregnated with the oil even after prolonged evacuation andimpregnation under pressure. It appears as if the fine pores resist theflow of the varnish or oil therethrough, the residual gases in the poresare not displaced by the impregnant but rather are compressed by theentering fluid and thereby this prevents the complete impregnation ofthe members.

It has been discovered that, if members being treated with liquidcompositions are heated by infra-red radiation to temperatures of above80 C. while subjected to a high vacuum and flooded with fluidcompositions and the temperature maintained during the flooding, untilbubbling indicating escaping residual gas ceases, unexpectedly rapid andsubstantially complete penc tration of the pores of such members isaccomplished by the fluid compositions.

The object of this invention is to provide for enabling the rapid andthorough impregnation of porous members with fluid compositions bysubjecting the members to simultaneous evacuation and heating byinfra-red radiation previous to impregnation.

, A further object of this invention is to provide for more completelyimpregnating porous members with insulating fluid compositions byevacuating them to a pressure of less than five inchesof mercury andheating them with infrared radiation to a temperature to at least C.,

then covering the members with a fluid composition and continuingheating in the presence of the fluid composition to at least 80 C.

Other objects of the invention will, in part, be obvious and will, inpart, appear hereinafter.

For a better understanding of the nature and objects of the invention,reference should be had to the following detailed description anddrawing in which the single figure is a vertical cross section throughan impregnating tank, illustrating the practice of the invention.

It has been discovered that the impregnation of porous members by fluidcompositions may be greatly expedited and rendered more complete bysubjecting the porous members to simultaneous evacuation to a lowpressure and irradiation with infra-red radiation to heat the members toat least 80 0., covering the members with the fluid composition whileevacuated, and heating the covered members while so covered to bringthem up to a temperature of at least 80 C., so that residual gases willbe almost entirely driven off at a rapid rate, as evidenced by extensivebubbling, and finally applying pressure on the composition after thebubbling substantially ceases. Heating members by infra-red radiationhas been found to be unusually effective in the process of thisinvention providing for the vacuum impregnation of porous members. Otheradvantages are that the infra-red radiation may be conducted withoutheating any extraneous parts of the vacuum equipment and it enables theheating of the porous members to be accomplished rapidly. Nearly allinsulatingroils and insulating varnish compositions, with the exceptionof heavily pigmented compositions, through which infra-red radiationwill not penetrate, effectively, are relatively transparent toinfra-red'radiation so that eificient heating of the members may becarried out While covered by such fluid compositions.

Referring to Fig. 1 of the drawing, there is illustrated one form ofapparatus for the practice of the invention. A hermetically scalablevessel H] is provided with a cover l2 provided with a window Hirelatively transparent to infra-red radiation. Numerous glasses for thispurpose are known. The vessel ii] is provided with a conduit l6 forintroducing and withdrawing fluid compositions with which members are tobe impregnated. A pipe I8 connected to a suitable source of vacuum (notshown), such, for example, as a vacuum pump, enables the vessel iii tobe evacuated to any desired degree. The pipe l8 may be also connected toa source of air, carbon dioxide or other gas under pressure, ifnecessary, for building up pressures above atmospheric, or else the pipeI8 may be opened to the atmosphere to break the vacuum. Within thevessel i is disposed a horizontal support 20 provided with a pluralityof bars 22 on which porous members 2 to be impregnated may be placed,arranged so that the fluid composition may readily penetrate between thebars 22. It will be appreciated that the bars 22 may be replaced bytheir equivalent, such as the grating, wire screen or the like. Exteriorof the vessel It! is located one or more infra-red lamps 26,constituting a source of infra-red radiation. While lamps have beenshown, it will be understood that electrically heated resistanceelements and other infra-red radiating means, preferably with focusingshields, may be employed. In some cases we have placed the infra-redlamp within the vessel Iii itself, though it is more convenient toaccommodate the focusing of the lamps 26 to the number and dispositionof the members 24 within the vessel if the lamps are exterior of thetank. In addition, the heat from the lamps other than the directedradiant heat is more conveniently removed, if the lamps are not withinthe vessel ill.

After the porous members 2e, which may be electrical coils or any otherelectrical pieces requirin impregnation with a fluid composition, aredisposed in suitable manner on the bars 22, the cover i2 is applied tothe vessel and the vessel hermetically sealed. It has been found thatthe best results are obtained if the members 24 are disposed in a singlethickness to form a horizonta-l layer on the bars 22. Likewise, if themember has openings or pores extending to one side only, as, forexample, in vibration pickups, it is preferable that the porous side beplaced uppermost. The vessel I0 is evacuated through the pipe !8 to asuitable low absolute pressure, generally below five inches of Hgabsolute. For best results, we have evacuated to an absolute pressure ofbelow mm. Hg. The pressures are exemplary and for very viscous fluid theabsolute pressure may be much lower. The evacuation removes air,moisture and other volatile material from the porous members 24. In somecases a substantial amount of gas and other volatilizable substanceswill remain in the fine interstices and pores. Thus, we have found thatcoils wrapped with eight to ten layers of mica and varnished tape retaina substantial amount of gas in the innermost layers thereof, which isnot removed even with drastic vacuum treatment.

Soon after evacuation is started, the infrared lamps 26 are focused onthe porous members 24 and put in operation so that infra-red radiationis applied to the members 24 to cause them to heat up. A temperature ofat least C. has been found to give the best results in combination witha suitable low vacuum. The combination of infra-red heating and thevacuum has been found to result in the expeditious elimination of allbut traces of gases and volatilizable matter in the porous members 24.

Inasmuch as infra-red radiant heating is capable of transmitting energyto the members at an extremely high rate, the members may be heated in avery short period of time, usually a few minutes is adequate. This isadvantageous securing prompt and thorough degassing. After the members24 have been heated and evacuated adequately, a fluid composition may beintroduced through the conduit l6 so as to cover the members 24. In thepractice of the invention, the height of the fluid composition 28 isadjusted to barely cover the members 24. This insures the maximumabsorption of the infra-red radiation by the members 24 and also thatthe least amount of the fluid composition is heated unnecessarily. Uponcontact of the hot members 24 with the fluid composition 28, they willbe cooled considerably since ordinariy the fluid composition is not ashot as they are. Particularly in the case of heat reactive insulatingvarnishes, preheating of the composition 28 would be undesirable sinceit would cause premature gelation and poor storage life of thecomposition.

Irradiation of the members by the infra-red radiation iscontinued duringthe immersion of the members in the composition 28 until the members areat a temperature of at least 80 C. in the case of varnish compositions.Much higher temperatures may be employed if the members 24 are to beimpregnated in an insulating oil, such as transformer oil or a siliconeoil. Bubbles of gas are given off by the porous members as they areheated.

To secure the best impregnation results, after the members have beenimmersed, heated to at least 80 C. and bubbling has ceased, the vacuumshould now be broken and either atmos pheric pressure is applied or apressure much higher than atmospheric pressure, may be imposed upon thefluid composition 28 by connecting the pipe l8 to a suitable source ofgas under pressure. Pressure of as much as pounds per square inch may beapplied to the fluid composition 28 to cause the composition to beforced into the finest and inner most interstices in the members 24.Within a short period of time, usually a few minutes, the members 24will be completely and thorough saturated with the fluid composition,and the fluid composition may be withdrawn through conduit I6 to uncoverthe members 26. The members may be permitted to remain on the bars 22for superficial drainage of residual composition. After suitabledrainage, the members may be removed and processed further. If aninsulating oil is applied, for'example, to impregnate a vibrationpickup, the pickup may be sealed while covered with oil and the sealedunit need only be wiped to be ready for use. Electrical coilsimpregnated with an insulating varnish will be removed and put in abaking oven or otherwise heated to cure the varnish therein.

Unexpectedly good results have been secured by the combination ofinfra-red heating and evacuation. It has been found that members, as,for example, a vibration pickup, cannot be completely and thoroughlysaturated with an insulating oil without any appreciable amount of gasor moisture being left, by the use of vacuum alone. Furthermore, thetreatment without the use of heat was more than three times as long withless satisfactory result than secured by a short treatment usinginfra-red heating combined with the vacuum treatment.

Numerous varnish compositions may be employed in the practice of thepresent invention for impregnating coils. It is preferable that thecompositions be relatively transparent to infrared radiation thereof.This requirement may be dispensed with if the members are barely coveredwith the fluid composition, that is, not over a fraction of an inch ofthe composition is over the surface of the members. Examples of suitablevarnishes for the practice of the invention are alkyd varnishes,phenolic resin varnishes, melamine resin varnishes, polyestercompositions embodying an unsaturated alkyd resin and a vinyl monomer.Specific examples of such polyester resins are disclosed in Patent No.2,414,525. Examples of suitable oils are transformer oil, castor oil,silicone oil and chlorinated diphenyl.

Since certain obvious changes may be made in the above procedure anddifferent embodiments of the invention could be made without departingfrom the scope thereof, it is intended that all matter contained in theabove description shall be interpreted as illustrative and not in alimiting sense.

I claim as my invention:

1. In the process of impregnating a porous member with a fluidcomposition, the steps comprising subjecting the member to a vacuum atan absolute pressure of less than 5 inches of Hg to Withdraw air,moisture and other volatiles from the pores thereof, irradiating themember with infra-red radiation to cause its temperature to rise above80 C. while still subjected to the vacuum, immersing the member with anamount of a fluid composition barely covering 7 the upper surface of themember while the member is maintained under vacuum, the fluidcomposition being relatively transparentto infra-red radiation,continuing heating the member with the infra-red radiation while it isso covered with the fluid composition until the member again reaches atemperature of at least 30 C. and continuing such heating until bubblingceases, and withdrawing the impregnated member from the body of thefluid composition.

2. In the process of impregnating a porous member with a fluidcomposition, the steps comprising subjecting the member to a vacuum atan absolute pressure of less than 5 inches of Hg to withdraw air,moisture and other volatiles from the pores thereof, irradiating themember with infra-red radiation to cause its temperature to rise above80 C. while still subjected to the vacuum, barely covering the memberwith a fluid composition selected from the class consisting ofinsulating varnishes and dielectric liquids while the member ismaintained under vacuum, the fluid composition being relativelytransparent to infra-red radiation, continuing heating the member withthe infra-red radiation while it is so covered with the fluidcomposition until all gas bubbling from the member ceases, and thenapplying pressure to the fluid composition about the member to cause itto penetrate into the member, and withdrawing the impregnated memberfrom the body of the fluid composition.

3. In the process of impregnating a porous member with a varnish, thesteps comprising placing the member in a hermetically sealable vessel,evacuating the vessel to an absolute pressure of below 5 inches of Hg towithdraw air, moisture and other volatiles from the member, irradiatingthe member with infra-red radiation to cause its temperature to riseabove C. while being subjected to the vacuum, immersing the member in avarnish relatively transparent to infra-red radaiation while beingirradiated and under vacuum, breaking the vacuum when the impregnatedmember reaches a temperature above 80 C. and ceases to give off gasbubbles, applying a pressure equal to at least atmospheric pressure tothe varnish to force it into the member, removing the varnishimpregnated member from the vessel and baking it to cure the appliedvarnish.

4. In the process of impregnating a plurality of members with a fluidcomposition, the steps comprising placing a single thicknes of themembers in a single horizontal layer in a hermetically sealable vessel,evacuating the vessel to an absolute pressure of below 5 inches of Hg towithdraw air, moisture and other volatiles from the pores of themembers, irradiating the layer of the members with infra-red irradiationto bring their temperature to at least 80 C. while being subjected tothe vacuum, immersing the layer with a fluid composition in an amount tojust cover the members, the fluid composition being relativelytransparent to infra-red radiation, continuing irradiating while soimmersed until the members reach a temperature above 80 C. and cease togive oil gas bubbles and withdrawing the impregnated members from thevessel.

5. In the process of impregnating a porous member with a fluidcomposition, the steps comprising subjecting the member to a vacuum atan absolute pressure of less than 5 inches of Hg to withdraw air,moisture and other volatiles from the pores thereof, irradiating themember with infra-red radiation to cause its temperature to rise above80 C. while still subjected to the vacuum, immersing the porous memberwith a fluid composition relatively transparent to the radiation in anamount barely covering the upper surface of the member while the memberis maintained under vacuum, continuing heating the member with theinfra-red radiation While it is so covered with the fluid compositionuntil all gas bubbling from the member ceases, and

withdrawing the impregnated member from the body of the fluidcomposition. HERBERT M. KROFT.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 577,671 Tesla Feb. 23, 18971,163,342 Hurley Dec. 7, 1915 1,760,583 Clark May 27, 1930 2,282,317Bennett May 12, 1942 2,321,939 Quinn June 15, 1943 2,357,286 ReavellSept. 5, 1944

